Testing of concrete

ABSTRACT

The strength of a brittle material, such as concrete, is tested by applying opposing forces on a ring of predetermined dimensions on the surface of the material and an area which is within the material and smaller than that of the ring, the forces being applied in the sense to pull out material against reaction on the ring. In practice the ring is defined by an annular member disposed on the material surface, and the small area by a boltlike insert which can be of simple unitary form embedded in cast material or expandible head form secured in a suitable bore and cavity in solid material, while the force is applied by a hydraulic ram coupling the insert and annular member. An important feature of the test is the predetermined containment of prospective material rupture by the ring, thus affording a controlled test which correlates well with conventional test results.

limited tales tent Feb, 3, 1972 Moshe Teeni, Southampton, England National Research Development Corporation, London, England [22] Filed: Aug. 4, 1969 [21] Appl.No.: 847,228

FOREIGN PATENTS OR APPLlCATIONS 240,077 9/1964 Austria ..73/88 E Primary ExaminerRichard C. Queisser Assistant Examiner-Marvin Smollar Att0rney--Cushman, Darby & Cushman [5 7] ABSTRACT The strength of a brittle material, such as concrete, is tested by applying opposing forces on a ring of predetermined dimensions on the surface of the material and an area which is within the material and smaller than that of the ring, the forces being applied in the sense to pull out material against reaction on the ring. In practice the ring is defined by an annular member disposed on the material surface, and the small area by a boltlike insert which can be of simple unitary form embedded in cast material or expandible head form secured in a suitable bore and cavity in solid material, while the force is applied by a hydraulic ram coupling the insert and annular member. An important feature of the test is the predetermined contain ment of prospective material rupture by the ring, thus affording a controlled test which correlates well with conventional test results.

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This invention relates to a method of testing the strength of brittle materials and more particularly, but not exclusively, concrete.

The conventional way of. determining the strength of concrete is by means of testing standard control specimens, such as cubes, prisms or cylinders, taken off the batch when the concrete is mixed. This method, however, is only capable of indicating the potential strength of the concrete in question since the curing conditions and degree of compaction are major factors affecting the strength of concrete and these may be different for the tested specimen than for the structure in which the batch of concrete is used.

An an alternative seeking to avoid this difficulty, specimens for similar testing are sometimes cut from the cast structure itself. However, the necessary cutting procedure is costly, and there can be an adverse effect on the structure resulting from the core material.

Some other methods of trying to determine the strength of the concrete in the structure are based on the measurement of some physical or mechanical properties which are related to the strength of the concrete, e.g., ultrasonic pulse velocity. These suffer from the variation in the correlation between the strength and the measured property for different concretes and under various conditions.

Finally, it is known to test concrete structures in situ by casting into the concrete a member having an annular surface parallel to the exterior of the concrete surface and a stem leading from the cast-in member to the exterior of the concrete. The member is then extracted by applying force to the stem which urges the cast-in member towards a reaction ring of annular configuration which is placed against the exterior of the building, etc., under test. The axial force applied to the stem to cause the concrete to shear gives a measure of the shear strength of the concrete. This method has the disadvantage that it cannot be utilized to test precase structures.

Of course, it is known to test the hardness of material by boring a cylindrical hole therein and then inserting therein a toggle-type reaction insert having a stem which is coaxial with the hole. When the reaction insert expands into the material due to axial force applied to the stem, this force may be measured to determine the hardness of the material.

Such an arrangement would not, however, be suitable even in combination with other known art to measure shear stress since it is essential to minimize the radial components of force applied to the concrete by the insert within the concrete in order to create a readily reproducible test.

The toggle insert used in hardness testing as well as the others known to the wall fastener and testing arts, which are designed for open cavities, produce radial as well as axial forces. This is, possibly, because the prior art could not readily produce a radially enlarged cavity in concrete and thus toggle linkages which could take advantage of such a wall surface, and thus produce a substantially axial force, were not useful, or, perhaps this was because of the superior grasping properties imported by a combination of radial and axial force. In any event, even where the prior art showed methods and apparatus for creating radially enlarged cavities, it showed fastening devices which produced radial as well as axial forces which are therefor inappropriate for measuring shear stress.

An object of the present invention is to obviate the abovediscussed difficulties in testing the strength of concrete.

To this end, the invention provides in a more general aspect, a method of testing the strength of brittle material, such as concrete, by applying opposing forces on a ring of predetermined dimensions on the surface of the material and an area which is within and smaller than that of the ring, the forces being applied in the sense to pull out material against reaction on the ring.

The ring is preferably substantially planar and of regular form, and in fact will normally be circular, and force is preferably applied at the small area in the direction of the axis of the ring.

In practice, the forces are applied between an insert passing into the material through the ring, and an annular member positioned on the ring.

The insert is preferably of a general form having a rodlike stem and an enlarged end portion for passage into the material, the enlarged end portion being of such configuration that the major part of a force applied axially away from such end can be transmitted to the material in the same axial direction. Putting this another way: lateral components of force applied to the material by the insert should be as small as possible. In case of an insert to be located in a bore, the bore is preferably complementary to the simple form just described with a substantially cylindrical bore terminating in a radially enlarged cavity, in which a wall is formed by a special drilling apparatus, radially disposed with respect to the bore axis, and facing the ring. Various insert forms having expandable end portions are described hereinafter for this purpose. All that is required of the inserts is that they be smaller, when contracted, than the main portion of the bore and that when expanded, they present a radial surface which rests against the wall of the cavity facing the ring.

The annular member can be of any suitable form, but preferably has a portion of regular cross section conforming to the reaction ring. if the material to be tested has, or is formed with, a flat surface the annular member defines the reaction ring directly. If, however, the material does not have a flat surface, it is preferred that the material be provided with an annular groove having a flat lowermost surface centered on the associated insert or insert bore. In the case of an embedded insert, such a groove can be formed during the casting procedure, or cut thereafter with the insert serving as a centering member; and in the case where a bore is used, the groove can be cut conveniently together with or in association with the bore by use of a common jig.

Turning to the question of force application: this may be effected by any suitable means, but is presently though to be best effected by hydraulic piston-and-cylinder means with the piston and cylinder components connected to respectively different ones of the insert and associated annular member. The latter means is preferred since it can be made available in robust and compact form, while permitting easy variation of applied force, application of large forces, and ease of force measurement and indication.

In order that the invention may be clearly understood, the same will now be more fully described by way of example with reference to the accompanying drawings, in which:

FIG. 1 schematically illustrates application of the present invention,

FIGS. 2 and 3 are graphs illustrating correlation between results obtained by use of the invention and currently standard tests,

FIG. 4 shows, in section, an expandable form of insert located in a preferred form of bore therefor in concrete,

FIG. 5 is a section of line V-V in FIG. 4,

FIG. 6 shows, in section, one example of a preferred form of expandable insert and associated apparatus,

FIG. 7 is a section on line VIIVII in FIG. 6,

FIG. 8 shows, in section, another example of a preferred form of expandable insert and associated apparatus.

Considering FIG. 1, this shows an insert ll of relatively simple boltlike form having its bolthead 2 embedded in concrete 3 by casting in situ with the bolt stem 4 projecting perpendicularly from the flat concrete surface 5. A right-circular-cylindrical annular member 6 is positioned with one end on the surface 5 coaxially around the bolt. The member 6 serves at its other end as the circumferential wall of a hydraulic cylinder 7 having end walls 8, a piston 9, and a piston rod 10 aligned with and extending towards the bolt stem 4. The bolt stem and piston rod are connected at ill by a threaded coupling or in any other suitable manner. The cylinder has two ports 2 for passage into the cylinder on respectively opposite sides of the piston, which ports are connected with a variable pressure hydraulic pump 13.

In carrying out a test according to the invention, fluid is applied at known pressure from the pump 13 to the underside of the piston 9 and exhausted from above the piston. This applies a known force between the bolt 1 and member 6, or rather a system of opposed forces as indicated by arrows, tending to pull out concrete from the mass 3 against reaction on the ring defined in the surface by the member 6.

The test may take one of two forms. Firstly, the test may be for a particular strength of concrete and this is effected by applying a predetermined value force which corresponds to such strength. If the concrete is up to the required strength, the force leaves the concrete intact; while if the required strength is not met, the concrete ruptures within the frustoconical zone indicated by chain lines l4 and defined by the head of the insert and the annular member ring. In practice the rupture occurs in the manner indicated by the broken lines which are curved slightly inwardly of the lines 14. The second form of test is that for maximum strength, in which the applied force is increased to a value at which rupture occurs as in the first test, and such force corresponds to a particular strength,

The important point to note in the tests just described is that the rupture, be it potential or actual, is of a controlled nature since it is contained by the reaction ring. It is this controlled nature of the tests which make them viable by affording a reasonably consistent correlation between applied force and material strength. Such a correlation has been established during initial development of the invention by comparison with standard control cube testing.

FIGS. 2 and 3 are examples of graphs plotted to show a useful correlation between standard tests and that of the invention, the latter having been carried out with simple apparatus involving embedded bolts as inserts. Improved apparatus has in fact now been developed and better results, showing more consistent correlation, are expected from further comparative tests now in progress.

In FIG. 2 the graph is plotted on the basis of linear scales between the pullout force P of the present test and the results T of corresponding 4-inch cube indirect tensile strength tests. The resultant graph indicates an optimum correlation on the basis of a curvilinear function as denoted by the broken line curve. However, this curve can be closely approximated in the region of practical interest by a linear correlation function of the form P=A Trt-B, where A and B are constants, as denoted in solid line.

In FIG. 3 the graph is plotted on the basis of logarithmic scales between the pullout" force P of the present test and the results C of corresponding 4-inch cube compressive strength tests. The resultant graph indicates a basis for a curvilinear correlation function as denoted in solid line and of the form P=LC N, where L, M and N are constants.

The tests for P in both FIGS. 2 and 3 involved use of conventional 74-inch bolts embedded in concrete to a depth of 2 inches and associated with a reaction ring of 6 inches diameter. Also, different concrete mixes and curing conditions were used.

In FIG. 4 the concrete structure 3 has a bore 16 formed in it at the position where it is desired to test the concrete. At the inner end of the bore 16 is a cavity 17 enlarged in diameter relative to the diameter of the bore 16, This bore and cavity can be formed by drilling the bore and then using an excavating type drill to enlarge the cavity at the inner end of the bore. While conventional excavating type drill equipment may be suitable for this last purpose, it is preferred that equipment involving an expandable water-cooled bit be used.

The insert 18, shown in FIGS. 4 and 5 consists ofa cylindrical body 19 having an axial bore 20 extending throughout its length, In the bore 2i) is a threaded rod 21 which is captive in the insert but can be rotated by applying a screwdriver to a slot 22 in the head 23 of the rod 21. At the lower end of the cylindrical body 19 are two slots 24 and 25 (see FIG. 5). The slots 24 and 25 are dimensioned and shaped to receive pivoted arms 26 and 27 connected by links 28, 29 to a nut 30. By rotating the rod 19 by means of a screwdriver the nut 30 can be moved up and down the threaded rod 29 so as to move the arms 26 and 27 from the position shown in solid line in FIG. 4 to the position shown in dotted line.

In use of the insert, the amis 26 and 27 will occupy the dotted line position initially while the insert is pushed into the bore 16 and, when the lower end of the insert is within the area of the cavity 16, the threaded rod will be rotated so that the arms 26 and 27 assume the solid line position. It will be noted that the arms 26 and 27 will prevent extraction of the insert during application of force against reaction through the annular member 6, since the arms will engage the upper wall 31 of the cavity immediately adjacent the inner end of the bore 16.

The insert 18 of FIG. 4 is given merely as an example of what can be termed a pivoted arm or toggle bolt type insert for use in the present invention. In addition, this example is shown with only two arms for the sake of simplicity, whereas it would be desirable to use a greater number than this in practice. The reason for this is that an expandable insert should apply force to the upper wall of the cavity as uniformly as possible a round the bore. The present thinking is that one would wish to apply force at at least six regions uniformly spaced around the upper wall of the cavity. This number of points is not intended to be construed as limiting the form of insert, but it does indicate that an insert such as that of FIG. 4 would be perhaps unnecessarily complex for practical purposes.

A preferred form of insert, which is of simpler form while affording more force application regions, comprises an expandable part in the form ofa longitudinally split sleeve structure supported around a stem and expandable by adjustment of a tapered member axially of the stem to provide a wedging action radially outwards of the sleeve. The tapered means can be provided in the form of a further sleeve drivable between the split sleeve and stem, or such means may be connected for movement with the stern itself.

One example of such an insert is shown in section together with associated apparatus in FIG. 6.

The insert 32 extends through an aperture 33 in a cylinder 34, the insert being slidable in O-rings 35 in the aperture 33. The insert 32 is attached to a hydraulic ram 36 operable by oil supplied through a pipe 37 from a pump, not shown.

The annular member 38 defining the reaction ring is connected to a sleeve 39 via splines 40. The sleeve 39 is threaded on to the insert 32, the threaded portions of the insert and sleeve being shown at 4i. The sleeve 39 has a tapered end 42 engageable beneath spring-loaded fingers 43 which are formed by longitudinal cuts in a sleeve and correspond to the radial arms in the previously described embodiment. The fingers 43 are normally loaded so that they remain in the vertical position shown in FIG. 6, but by rotating the annular member 38 by means of a lever 44, the sleeve 39 will be rotated and will travel vertically downwardly, so that the tapered end 42 engages the fingers 43 and causes them to expand outwardly. The upper ends of the fingers are thus positioned below the upper wall 45 of the cavity 46, and the hydraulic ram can then be operated, so that the insert 32 tends to be pulled out of the concrete as described above.

FIG. 7 is a section on the line VII-VII in FIG. 6 and illustrates number and regular disposition of the fingers 43 around the central stem of the insert.

FIG. 8 illustrates yet another example of the preferred form of insert 47 and associated apparatus.

The insert 47 comprises a central stern in the form of a cylindrical rod 48 terminating at its lower end in a divergent frustoconical part 49 and a disc part 50 of larger diameter than the cone base, so providing a shoulder 51. An upper part 52 of the stem is of reduced diameter and threaded. The insert also comprises a longitudinally split sleeve 53 having outwardly extending flange portions 54 and 55 at its upper and lower ends, respectively. The sleeve 53 can be regarded as having a bore to slide over the rod 4-8 with its lower end resting on the frustocone 49, and an outer surface which convergently tapers towards flange 55. The dimensions of the various inserts may differ within a wide range, however, the relative dimensions between a given insert and its reaction ring and the depth of a given bore should be similar to those described with respect to the embedded bolt.

The associated apparatus comprises a master-and-slave piston-and-cylinder assembly within a cylinder block 56. The master cylinder 57 and the piston 58 of its piston 59 are threadably engaged for manual operation thereof by a screw action with handwheel 60. Hydraulic fluid acted upon in this way communicates through a passageway 61 with the slave cylinder 62 and its piston 63. The piston 63 is bored and threaded to receive an externally threaded sleeve 64 which can be screwed relative to the piston by a handwheel portion 65 of the sleeve.

The annular member associated with the insert 47 is in two parts, namely, an upper part 66 adapted to support the cylinder block 56 and a lower part 67 which defines the reaction ring, the parts 66 and 67 being of threadably engageable, generally sleeve form.

Other parts of the associated apparatus comprise a collar 68 and a handwheel/nut 69 for engagement with part 52 of the insert. Also, a hydraulic fluid pressure of force indicator 70 is coupled to a branch from passageway 61, this indicator being preferably of a kind to retain, until released, a maximum indication during use.

Use of the insert and associated apparatus of FIG. 8 involves provision of a bore with enlarged cavity in material to be tested as indicated in chain line. The annular member and cylinder block are positioned on the material surface, as shown, with the master piston bore coaxially aligned with that of the bore in the material. Sleeve 64 is screwed into the slave piston 63 by handwheel 65 and the insert is passed through this sleeve and piston into the material bore. The lower end of the insert is positioned in the material cavity, while the upper end flange 54 sits on the sleeve 64 which may be provided with a shoulder for this purpose. The annular member is adjustable to match the insert length and bore depth in this connection, as it were. The collar 68 is then slipped over and the nut 69 screwed on the insert upper end to draw the frustocone 49 into the split sleeve 53 and so expand the insert lower end until this is engaged by the shoulder 51 of disc 50, as shown. The handwheel 65 is then rotated to withdraw the sleeve 64, and with it the insert, outwardly of the slave piston 63 until the lower flange 55 engages the upper wall of the cavity.

Finally, the handwheel 60 is rotated to drive the master piston 59 into its cylinder 57 and so apply force on the insert 47, by way of the slave piston 63, sleeve 64, collar 62 and nut 69, tending to pull out material against reaction through the member 66, 67. The indicator 70 serves to indicate the maximum force to which the material is tested or that at which the material ruptures.

This form of apparatus is advantageous for the reasons discussed earlier and also for its adaption to ease of manual operation. The latter advantage arises from the use of screw actions with handwheel leverage and the master-and-slave hydraulic system giving high mechanical advantages.

Regarding the insert 47: this can involve a split sleeve 53 in the usual sense of an integrated component with its fingers connected in the region of flange 54, or it can involve a number of like finger components which can be assembled in the form of such a sleeve. The former is advantageous in affording greater ease of location in use, while the latter affords economy of replacement if part of the sleeve breaks.

The present invention has been more particularly described with reference to testing the strength of concrete since it was for this purpose that the invention was initially developed. However, the invention is not intended to be construed as limited to such testing. The invention can clearly be applied to other materials which will fail in a similar manner to concrete when subject to such testing, and an immediate example in this connection is natural rock which can be tested for foundation stren th.

Also, t e invention lS not limited to onsite use for civil and structural engineering purposes as might be implied by reference to concrete and rock foundation. The invention may find use in laboratory testing of materials such as ceramics, for example.

in general then, the invention is applicable to what are termed brittle materials.

I claim:

1. A method for testing the shear strength of brittle material which comprises the steps of:

a. boring a substantially cylindrical hole perpendicular to the exterior surface of said material,

b. forming an enlarged cavity toward the inner end of said bore, said cavity having an annular face nearer the exterior surface of said material extending radially relative to the axis of said bore,

c. placing an annular reaction member having a substantially planar surface on the exterior surface of said material coaxial with and surrounding said bore,

d. inserting a stern into said bore, said stem having a head with a radially contracted and a radially extended position said head being inserted in its radially contracted position,

e. opening said head to its radially extended position,

f. placing said head in contact with said annular face of said cavity,

g. applying an axial force to said stem opposed by said reacting member which tends to remove a portion of said material, and

h. measuring the force applied to said stem.

2. A method according to claim 1 which comprises applying said forces up to predetermined value corresponding to a required strength for the material.

3. A method according to claim l which comprises applying said force in increasing manner up to a value at which the material ruptures. 

1. A method for testing the shear strength of brittle material which comprises the steps of: a. boring a substantially cylindrical hole perpendicular to the exterior surface of said material, b. forming an enlarged cavity toward the inner end of said bore, said cavity having an annular face nearer the exterior surface of said material extending radially relative to the axis of said bore, c. placing an annular reaction member having a substantially planar surface on the exterior surface of said material coaxial with and surrounding said bore, d. inserting a stem into said bore, said stem having a head with a radially contracted and a radially extended position said head being inserted in its radially contracted position, e. opening said head to its radially extended position, f. placing said head in contact with said annular face of said cavity, g. applying an axial force to said stem opposed by said reacting member which tends to remove a portion of said material, and h. measuring the force applied to said stem.
 2. A method according to claim 1 which comprises applying said forces up to predetermined value corresponding to a required strength for the material.
 3. A method according to claim 1 which comprises applying said force in increasing manner up to a value at which the material ruptures. 